As the blow-molded hose of the synthetic resin, Japanese Patent Laid open No. 14445/1996 (JP-A-8-14445) discloses a blow-molding pipe or hose. The blow-molded hose is prepared by blow-molding a parison having a laminating structure of a soft resin layer and a hard resin layer with varying partially the thickness ratio of the soft and hard resin layers in the longitudinal direction in order to form mutually a flexible soft portion and a rigid portion for shape reinforcement. However, when the flexible soft portion is formed at a joint or connection edge of the duct or hose for connecting with other parts or members, the flexibility of the soft resin portion is deteriorated by the hard resin since the hard resin layer is laminated on the soft resin layer irrespective of the thickness ratio. Therefore, the joint or connection characteristics with the other members such as a metal pipe are lowered and the high sealing at the connection site cannot be realized.
A multilayer (multi-walled) hose or tube is manufactured by extruding molten resin in the form of a parison from an annular die slit, setting the parison in a mold, and carrying out blowing. In this molding operation, molten resin from a resin feeding side-port flows down a circumferential annular passageway and is extruded in an axial (downward) direction from an annular die slit. Therefore, not only the flow and flow rate of molten resin are complicated but also a large pressure disparity between the part corresponding to the resin feeding port and the part opposed to the feeding port, as well as a pressure strain, is created to cause variations in thickness, with the result that a hollow body of uniform wall thickness can hardly be obtained. Particularly in the extrusion-blow molding of a multi-walled hollow product, such a variation tends to occur more readily in each constituent layer.
To correct for such variations in thickness of the parison and hollow product, it is common practice to displace the die in a horizontal direction with an adjusting bolt so as to change the slit dimension between the die and the mandrel. Japanese Patent Laid-open No. (JP-A-)51-134758 discloses an extrusion molding technology for the production of a multi-walled hollow tube wherein while the thickness ratio of constituent layers of the multi-walled structure is controlled by judicious setting of the co-extrusion speeds of the ring piston, the exit width of the resin passageway downstream of the point of convergence of resin streams for respective layers is adjusted by the vertical displacement of the mandrel or the horizontal displacement of the die so as to control the wall thickness. This literature further teaches the utilization of the horizontal displacement of the die for offset control.
Japanese Patent Publication No. (JP-B-)3-5286 discloses a prior art technology which comprises constituting a die with a flexible ring and pressing both lateral sides of the die with actuators to deform the die into an elliptical shape to thereby adjust the annular slit between the die and the mandrel and correct for a variation in thickness of the parison.
However, since the points of variation in thickness of a parison are not diametrically opposed, it is still difficult to obtain a parison and hollow tube of uniform wall thickness even if the thicknesses at opposed points are adjusted. Particularly in the production of a multi-walled hollow product, thickness variations may occur respective constituent layers so that the thickness of the parison cannot be controlled with accuracy.
Furthermore, in the intermittent production of parisons and hollow tubes using an extrusion-blow molding machine, the variations in thickness result in resin leaks through the clearance between the die and the mandrel. Thus, the intermittent production of parisons and hollow tubes comprises raising the ring piston to close the annular slit between the die and the mandrel, collecting a parison-equivalent amount of molten resin in the accumulation chamber, driving a rod of the mandrel forward to form an annular slit, lowering the ring piston to expel the molten resin from the accumulation chamber to form a parison having a wall thickness corresponding to the slit dimension, and driving the rod and mandrel back to close the annular slit, all as a cycle on an intermittent basis.
Such being the process, when the rod is driven back after the above thickness adjustment of the parison by the adjusting bolt, the annular slit cannot be hermetically closed by the mandrel because of the offset of the die with respect from the mandrel so that a gap typically as wide as about 1 mm is formed between the die and the mandrel. Then, as molten resin is collected in the accumulation chamber with the annular slit having been closed by the mandrel, the resin leaks out from the gap. Therefore, in the long-time repeated production of many parisons and hollow products, a large waste of resin takes place to detract from effective utilization of the resin and economics of production.
Furthermore, as the rod is driven back with a strong force with the die offset from the mandrel, a bending force, instead of an axial force, acts on the rod along the taper of the die and the mandrel to bend the rod and injure the joint between the mandrel and rod. Therefore, the rod should be driven back as gently as possible to let the mandrel be abutted against the head.
Moreover, according to the above operation, a monolayered flexible segment or rigid segment and a laminated segment of the hose or tube can hardly be constructed with high precision because the resin leakage cannot be completely prevented.
Japanese Patent Publication No. (JP-B-)52-37026 discloses an extrusion molding apparatus for multi-walled tubes which, for the purpose of preventing variation in thickness and bending of the parison without adjusting the clearance between the die at the tip of a head and the mandrel, comprises a cylindrical wall element having a temperature regulating function disposed in a laminar arrangement to provide a plurality of cylindrical resin accumulation chambers within a blow-molding head, an extruder for extruding a resin into the accumulation chambers, and a slidable ring piston for expelling the resin from the accumulation chambers. Japanese Patent Publication No. (JP-B)57-53175 discloses an extrusion molding apparatus comprising a molding head having a die and an annular passageway, a cylindrical partitioning wall and a ring piston as disposed in a laminar, vertically slidable relation in the molding head so as to form a cylindrical accumulation chamber, and an extruding means for extruding a polymer from the cylindrical accumulation chamber. It is taught in these literature that the ring piston is raised (driven back) to prevent leakage of resin from the clearance between the die and the mandrel.
However, since the resin is aspirated into the accumulation chamber by driving the ring piston back so as to prevent leakage, it is necessary that an excess or more than one parison-equivalent of resin should be accumulated in the accumulation chamber. Therefore, the excess of resin in the accumulation chamber is also extruded in the formation of a parison, so that the desired effective utilization of resin cannot be materialized.